The present invention relates to an apparatus for growing single crystals, which can prevent semiconductor single crystals from being contaminated by carbon or heavy metals during a drawing operation, and which further reduces crystal defect density within the grown semiconductor single crystals.
Known as one of the methods for growing single crystals of a semiconductor material such as silicon (Si) or gallium arsenide (GaAs) is a continuous Czochralski method which is a modification of a conventional Czochralski method. The continuous Czochralski method involves growing semiconductor single crystals while introducing raw material continuously into an outer crucible of a double crucible assembly, and is regarded as being the most suitable for growing long single crystals of large diameter.
A crystal growing apparatus to be used for carrying out the aforesaid continuous Czochralski method comprises a hermetically sealed container which is filled with an inert gas at a low pressure, a double crucible assembly of quartz (SiO.sub.2) including inner and outer crucibles and placed in the container, a heat-resistant support member for releasing the inner crucible from the outer crucible and retaining the same above the outer crucible, and a heater for heating a semiconductor melt contained in the double crucible assembly.
In view of machinability and heat resistance, the aforesaid support member is suitably formed of a material having a high melting point such as molybdenum (Mo), tungsten (W), niobium (Nb), tantalum (Ta) or the like. Additionally, silicon nitride (Si.sub.3 N.sub.4) or silicon carbide (SiC) may be used as the material for the double crucible assembly, and in that case, the support member may be formed of amorphous carbon (graphite).
For growing semiconductor single crystals using the aforesaid double crucible assembly, a prescribed amount of a semiconductor raw material is first introduced in the double crucible assembly, and an inert gas such as argon gas is introduced into the hermetically sealed container. Thereafter, while rotating the double crucible assembly, the semiconductor raw material is heated above a temperature of growth of single crystals and melted completely. Then, a seed crystal is immersed in the semiconductor melt and pulled vertically upwards while rotating the crucible assembly, whereby a semiconductor single crystal is grown.
Furthermore, as the crystal grows, a new charge of the semiconductor material of a prescribed amount corresponding to the growth of the single crystal, i.e., pulled amount, is continuously furnished into and melted in the outer crucible and transferred to the inner crucible.
In the aforesaid apparatus of single crystal growth, however, it has recently been found that the semiconductor melt or grown single crystals may be contaminated by the substances from which the support member is formed. More specifically, when growing silicon single crystals, acidic silicon oxide (SiO) vapor is produced from the silicon melt and graphite reacts therewith to produce carbon monoxide (CO), and this carbon monoxide causes lattice defects within the silicon single crystals which are being pulled. Furthermore, the material with high melting point such as molybdenum or tungsten is oxidized by the acidic SiO vapor, and the resulting oxides are captured in the silicon single crystals and cause minute crystal defects which are referred to as stacking faults.
The aforesaid contaminations by carbon or heavy metals have not been previously taken into consideration. However, due to the recent demands towards higher purity and uniform quality, the adverse effects of the contaminations on the characteristics of the silicon single crystals have become crucial and nonnegligible.